1. Field of the Invention
The invention relates to an interface circuit for coding analog signals for digital transmission and decoding digital signals for analog transmission.
2. Discussion of Related Art
The interface circuit is of the type comprising a processing unit, an analog port having a pair of terminals for connection to an analogue loop, a digital output port and a digital input port, said processing unit including an input path between the digital input port and the analog port arranged to convert digital input signls to analog output signals, an output path between the analog port and the digital output port arranged to convert analog input signals to digital output signals and a cross-over path for supplying digital signals derived from analog input signals to the input path wherein the processing unit is arranged to synthesize a component of a terminating impedance across the analogue terminals.
An interface circuit of the aforesaid type is shown in United Kingdom Patent Application 2 086 196 which discloses a circuit for coding an analog signal from a telephone line. The resulting digital signal is processed in combination with an incoming digital signal by means of a digital filter. The output from this filter is then decoded and supplied to the telephone line via a transconductance amplifier. A problem with this circuit is that it may become unstable due to the delay introduced by the analog to digital converter, the digital filter and the digital to analog converter.
Another interface circuit is disclosed in the Ericsson Review of 1983, No. 3, pages 186-191 in which a digital filter synthesizes a terminating impedance in combination with a resistor placed across the analog terminals. This arrangement is suitable for providing a fixed terminating impedance but does not solve the problem of providing a variable terminating impedance particularly if a range of values are desired for which both the real and imaginary components are to be changed. A problem is that in attempting to provide stability for such a circuit it is difficult to maintain the necessary frequency response for each of the available terminating impedances. Thus even if a relatively small number of terminating impedances are available, complex filters must be added to maintain the necessary frequency response.